1. Field of the Invention
The present invention relates to a method for selectively coating activated surface portions of a plastic substrate with a temporary protective film.
2. Description of Related Art
As used herein, an activated plastic surface is a surface of a plastic substrate that has been modified to include chemical functional groups which improve the adhesion properties of the plastic substrate. An activated plastic surface has a higher free energy than a corresponding untreated or unactivated plastic surface.
The benefits of activating the surfaces of plastic substrates are well known, and various surface activation methods, such as plasma treatment, ozone treatment, flame treatment, fluorination treatment, sulfonation treatment, and electrical discharge treatment (e.g., corona treatment) are taught in the prior art. These activation methods generally alter the surface chemistry of the plastic substrate such that it contains polar functional groups. The polar functional groups render the substrate more receptive to paints, adhesives, foams and the like.
A problem with many activated surfaces, however, is that they can be easily abraded or disrupted by normal handling and shipping. Treated plastic parts exhibit poor adhesion properties in areas where the activated surface is disrupted. Therefore, to preserve the activated surface prior to painting or other post-activation processing, some manufacturers have designed carrying systems for the treated parts in which manual handling of the parts is avoided. These systems add to the cost and complexity of the overall manufacturing process. In addition, they do not enable the parts to be shipped to other facilities for subsequent processing; any painting or other post-activation processing must be carried out in the same manufacturing facility as the activation step. Given these drawbacks of the prior art, it would be desirable to provide a simple, low-cost method for temporarily protecting the activated surface portions of plastic substrates from damage during shipping and handling.
Methods for temporarily protecting unactivated plastic substrates from soiling are known. One such method, disclosed in U.S. Pat. No. 5,418,006, includes applying a temporary coating to an unactivated surface of a plastic substrate prior to exposing the substrate to contaminants. Following exposure of the substrate to contaminants, the temporary coating is removed, leaving a clean substrate surface. In order for the temporary coating to wet-out on the unactivated surface of the plastic substrate, it must contain a significant amount of a wetting or dispersing agent. Such a coating would not generally be useful for temporarily protecting an activated substrate surface since wetting/dispersing agents tend to be difficult to remove from activated surfaces. Residues left on an activated surface can interfere with subsequent processing of the substrate. Moreover, coatings containing significant amounts of wetting/dispersing agents are undesirable for use with the present invention since it is an object of the invention to provide a method for readily distinguishing between activated and unactivated portions of substrate surfaces. A coating containing a significant amount of a wetting/dispersing agent would wet out on unactivated portions as well as activated portions of a substrate surface.